The present invention relates generally to an electric actuator apparatus and, in particular, to an apparatus for actuating vehicle all-wheel steering systems.
Early all-wheel steering systems were generally applied to agricultural and construction vehicles. Since such types of vehicles typically are provided with hydraulic systems, the steering system was powered by hydraulic actuators.
Early four-wheel steering systems for automotive type vehicles steered both the front and rear wheels through steering gear mechanisms. The transfer function between the steering input produced by the driver and the front wheel steering angle, and the transfer function between the steering input and the rear wheel steering angle were both determined by the steering gear mechanism. However, in U.S. Pat. No. 4,705,131, a vehicle steering control system is shown in which the transfer functions for the front wheel steering angle and the rear wheel steering angle are different so as to obtain optimum cornering characteristics for the vehicle. This steering system has a steering input sensor, a vehicle speed sensor, means for determining the transfer function for the front wheels from a steering frequency and a vehicle speed, means for determining the transfer function of the rear wheels, a front wheel actuator for steering the front wheels, and a rear wheel actuator for steering the rear wheels. The means for determining the transfer function of the rear wheels may be an electronic circuit, or a hydraulic fluid flow restrictor such as an orifice, or mechanical gearing.
In U.S. Pat. No. 4,719,981, a four-wheel steering system includes a steering wheel, a front wheel turning mechanism for turning the front wheels in response to the operation of the steering wheel, and a rear wheel turning mechanism which turns the rear wheels in response to operation of the steering wheel and is provided with an electric actuator for changing the position of a moveable member the position of which determines a rear wheel turning angle ratio. The rear wheel turning angle ratio is the ratio of the turning angle of the rear wheels to the turning angle of the front wheels for a given turning angle of the steering wheel. A control circuit receives a vehicle speed signal from a vehicle speed sensor and controls the electric actuator according to the detected vehicle speed so that a target rear wheel turning angle ratio determined in advance according to the vehicle speed is obtained. A preset vehicle speed detector detects a preset vehicle speed and a rear wheel turning angle ratio detector detects the actual rear wheel turning angle ratio. A correction circuit receives signals from the preset vehicle speed detector and the rear wheel turning angle ratio detector and controls the electric actuator to change the position of the moveable member so that the actual rear wheel turning angle ratio coincides with the target rear wheel turning angle ratio at the preset vehicle speed.
U.S. Pat. No. 4,842,089 discloses a rear steering control system employing separate closed-loop feedback control and open-loop feedforward control paths. The response characteristics of the open-loop and closed-loop control paths are separately scheduled to optimize the overall system performance. A computer based control unit controls separate electric motors for turning the front and rear wheels.